1. Field
The embodiment relates to a radio apparatus and a configuration of an antenna in the radio apparatus.
2. Description of the Related Art
Along with the popularization of radio communications in recent year, various radio communication apparatuses (such as cellular phones, cordless telephones, wireless communication PC cards, small radio devices, and mobile radio apparatuses) have been in wide use. Among those apparatuses, small radio devices particularly are desired to have an antenna installed in their cases. For that reason, in order to realize reduction in size, weight and thickness, a configuration in which antennas are formed utilizing circuit boards.
Patent Document 1 (Japanese Patent Application Publication No. 08-204433) that is one of the known arts describes a configuration in which antennas are formed on a dielectric substrate. A pair of a microstrip line and a GND line are formed on the front surface and back surface of the dielectric substrate. On the front surface of the dielectric substrate, an antenna element is formed at the end of the microstrip line, and on the back surface of the dielectric substrate, another antenna element is formed at the end of the GND line. These antenna elements constitute a dipole antenna. Note that the configuration in which an antenna is formed utilizing a circuit board requires only a few number of components and a few mounting processes, and therefore the implementation time can be reduced.
The conventional antenna did not have a sufficient resonant frequency bandwidth. In particular, for the WiMAX that is expected to be widely popularized in the future radio communications, an antenna with a wide resonant frequency band has not been developed. Here, the resonant frequency band is not specified, but is for example defined by a range of a frequency band in which VSWR (Voltage Standing Wave Ratio) is smaller than 2.
When the resonant frequency band of an antenna is narrow, the antenna characteristics could be degraded only by a small external factor. For example, when a metal conductor is present near the antenna (e.g. placing a radio apparatus on a metal table), the resonant frequency band of the antenna may be changed. In such a case, the frequency of a desired wave becomes out of the resonant frequency band, with the result that the desired wave cannot be received. In order to reduce the size of the radio apparatus, an antenna has to be arranged near a high-frequency circuit. In such a case, the antenna characteristics are subjected to the influence of noise from the high-frequency circuit. For that reason, preventions such as a shield cover are implemented. At that time, if the resonant frequency band of the antenna is narrow, the required antenna characteristics sometimes cannot be obtained and transmission characteristics/reception characteristics are easily degraded. In order to secure desirable transmission characteristic/reception characteristics, large current is required to amplify signals, which would cause a problem in reducing power consumption of the radio apparatus.